Perl: modify the original file

Most of the time, we want to modify the content of the original file. Although the powerful text editing command sed exists in Linux, however, he can only generate new files or print them to the screen (perhaps sed also has the ability to modify the original file, which is not easy to learn. Please let me know ), but in Perl, there is such a command $ ^ I
Example:
#! /Usr/bin/perl
Use strict;

$ ^ I = "";
While (<> ){
S/^ rate. */rate = 0 /;
S/^ testmode. */testmode = 0 /;
Print;
}

$ ^ The value of I can decide whether to rewrite the file or backup file. to rewrite the file, you only need to set the value to null. to back up the file, then $ ^ I = ". bak ", he will name the modified file filename. bak now